So much depends on the meta game and what you have difficulty dealing with in your battles. The presence of skink poison shooting and S5 TG means I can deal with anything with high T but poor AS. The Krox are good against high T or high AS and fast flankers but striking last means that some models often die before getting to hit back. Sometimes that means taking lore of metal or lore of death on the slann to deal with those pesky high armour saves and give me more ability to realize magical attacks. Plus final transmutation is wonderful spell for dealing with hordes, especially white lion units with BOTWD. You just can't count on getting spells off with the new slann like you could with the old slann with cupped hands and the +1 PD on each casting attempt.
If you expect to see a lot of high armour save armies (warriors of chaos, empire cav and stank, maybe brett cav) and see that is the biggest hole to fill in your army, then the great weapon or +1S magic weapon is in order to cut through the high armour saves. The great weapon OTS is particularly effective in dealing with high elf characters and units with boosted ward saves from High Magic spells and those 3+ ward save warriors of chaos characters, esepcially when the oldblood is already in a stubborn unit of TG. On the other hand, an oldblood led saurus unit becomes very effective and putting the crown of command on the old blood can be huge in allowing that unit to stick around for a long time, especially if you conga line the unit.
If you see a lot of vampire armies, you might consider a magic weapon for dealing with ethereal units/characters and use the shield to get to the 1+ AS and that will cut down on the armour of destiny, dawnstone plus OTS or crown of command combinations.
If you do not expect to see a lot of cannons and template hits (like stone throwers and spells), then putting the oldblood on a cold one saves a lot of points and gives you immunity to stomps and fear plus two more S4 attacks albeit at the price of taking stubborn tests. However, I think the scar vet gains more from the cold one's +2 AS boost.
Another consideration is whether you want a scar vet as well. A scar vet with dawnstone is quite potent when facing less than S6 attacks. That might mean only taking luckstone on the oldblood with armour of destiny. Similarly, a scar vet with magic heavy armour and a 5+ ward or 4+ ward is quite effective, especially on a cold one. So, one has to balance out how one wants to use those key common magic armour and talismen items. Sometimes gaining +1 T with the sacred stegadon helm is as good as gaining a 5+ or 4+ ward save or having a re-rollable 1+ AS in terms of total wounds a model will take in combat if one expects to mostly see S3, S4 and S5 opposing characters and units and can pick one's fights with the oldblood and then save the armour of destiny for a scar vet.
Also, if you take OTS, the oldblood should avoid being in base contact with the slann and any other model with a ward save.
This is pulled from something else I did:
Sometimes, I find it useful to look at kitting out one oldblood and one scar vet by simply listing out all the options with the points costs recognizing that some options do not count toward magic items limits. This is especially important if one is considering how to balance points on a slann with an oldblood in the army and whether two scar vets instead of one oldblood might be a more points efficient option. While the LD, I , T and S is the same for the scar vets as for oldbloods, sacr vets' lower magic points limit lreduces their ability to help the unit (with crown of command) and survive and hit back effectively. Oldbloods do get +1 wound, +1AS, +1 A and +1 WS and that sometimes ends up being really important in challenges and in being able to hit back effectively. Think about it this way: an oldblood with a great weapon, armour of destiny, and dawnstone or great weapon with steg helm plus light armour plus talimen of preservation against S4 or S5 attacks will survive as much as 3 to 12+ times longer than a kitted out scar vet can survive in combat and often will hit better, harder and for more rounds of combat with +1WS, +1A and more ability to take a great weapon or magic weapon. I find scar vets better cowboy models because the +2AS of the cold one is more needed on a scar vet to get to 1+ AS effectively and extra attacks of the cold ones pay off and I can afford to lose one to a cannon or template hit, whereas oldbloods tend to go to the heart of the army and I am less willing to risk their points to cannons and templates and magic picking them off.
Most Commonly Used Magic Armour and Armour Save Options:
Sacred Steg Helm
Armour of Destiny
Armour of Fortune
Dragonhelm
Charmed Shield (if on carmosuar or solo cold one)
Enchanted Shield
Light Armour
Shield
Cold One Mount
Most Commonly Used Talismen:
Dawnstone
Luckstone
Talismen of Preservation
Talismen of Endurance
Opal Amulet
Most Commonly Used Enchantments:
Other Trickster's Shard
Crown of Command
Ironcurse Icon
Healing Potion
Potion of Strength
Potion of Toughness
Egg of Quango
Most Commonly Used Weapons:
Handweapon
Additional Hand Weapon
Great Weapon
Biting Blade
Sword of Might
Gold Sigil Sword
Sword of Striking
Sword of Swiftslaying
Obsidian Blade
Piranha Blade
Sword of Antiheroes